Mobile communication systems have been developed because there has been a need to allow people to move away from fixed telephone terminals without losing their ability to be reached. While the use of different data transmission services in offices has increased, different data services have also been introduced into mobile communication systems. Portable computers enable efficient data processing whereever a user moves. Mobile communication networks, provide a user with efficient access network to actual data networks for mobile data transmission. In order to do this, different new data services are designed for existing and future mobile communication networks. Digital mobile communication systems, such as the pan-European mobile communication system GSM (Global System for Mobile Communication), support particularly well mobile data transmission. General Packet Radio Service (GPRS) is a new service in the GSM system, and it is one the items of the standardization work of the GSM phase 2+ in ETSI (European Telecommunication Standard Institute). The GPRS operational environment consists of one or more sub-network service areas, which are interconnected by a GPRS backbone network. A sub-network comprises a number of packet data service nodes, which are referred to as GPRS support nodes (or agents) in this context, each packet data service node being connected to a GSM mobile communication network so that it is capable of providing packet data service for mobile data terminal equipment via several base stations, i.e. cells. The intermediate mobile communication network provides circuit switched or packet switched data transmission between a support node and mobile data terminal equipment. Different sub-networks are connected to an external data network, such as a public switched packet data network (PSPDN). The GPRS service thus produces packet data transmission between mobile data terminal equipment and external data networks with a, a GSM network acting as an access network. The GPRS service network operates almost independently of the GSM network. One of the requirements of the GPRS service is that it must operate with external PSPDNs of different types, for instance, with Internet or X.25 networks. In other words, the GPRS service and a GSM network should be capable of serving all users, irrespective of which type of data networks they want to register in via the GSM network. This means that the GSM network and the GPRS service have to support and handle different network addressing and data packet formats. This handling of data packets also comprises routing in a packet radio network. In addition, users should be able to roam from a GPRS home network into an extraneous GPRS network, the backbone network of the operator of which may support a different protocol (for instance, CLNP) than the home network (for instance, X.25).